There are several different techniques for producing optical fiber for use in communications. One such technique comprises directing a constantly moving stream of reactants and oxygen through a glass substrate tube having a generally circular cross-section. The oxygen stream carries silicon tetrachloride and dopants to produce the desired index of refraction in the finished optical fiber. The substrate glass is heated to a reaction temperature within a moving hot zone that traverses the length of the tube, and the consequent reaction produces doped silicon dioxide fused into a continuous layer on the inner wall of the tube. The resulting tube is referred to as a preform tube. See for example, U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980 in the names of J. B. MacChesney and P. B. O'Connor. Subsequently, the preform tube is collapsed to provide a rod-like preform from which optical fiber is drawn.
A torch assembly, which typically is metallic, for heating a glass substrate tube to facilitate deposition of the reactants in the above-described process is disclosed, for example, in U.S. Pat. No. 4,231,777 which issued on Nov. 4, 1980, in the names of B. Lynch and F. P. Partus. See also U.S. Pat. No. 4,401,267 which issued on Aug. 30, 1983 in the name of C. D. Spainhour. Initially, one end of the tube is supported in the headstock of a lathe and the other end is welded to an exhaust tube that is supported in the tailstock. Combustible gases are directed through a housing and gas outlets which open to an arcuate surface of the torch assembly and toward the tube as it is turned rotatably about its longitudinal axis and as the torch assembly is moved therealong on a carriage to produce a moving hot zone. A temperature profile is produced across the hot zone which moves along on the surface of the tube to accomplish the desired reaction and deposition. See F. P. Partus, and M. A. Saifi "Lightguide Preform Manufacture" beginning at page 39 of the Winter 1980 issue of the Western Electric Engineer.
During a deposition mode, the torch carriage is moved slowly from the headstock of the lathe where dopants are moved into the glass tube to the tailstock where gases are exhausted. At the end of each pass from headstock to tailstock, the torch carriage is returned rapidly to the headstock for the beginning of another cycle. Ends of the gas outlets are cooled generally by water to eliminate substantially degradation by oxidation or reduction, for example, of the material forming the housing and gas outlets.
Subsequent to the deposition mode, a collapse mode is used to cause the preform tube to become a solid rod-like member which is called a preform. It is this preform from which lightguide fiber is drawn. See D. H. Smithgall and D. L. Myers "Drawing Lightguide Fiber" beginning at page 49 of the hereinbefore identified Winter 1980 issue of the Western Electric Engineer.
In order to collapse the preform tube, generally the torch assembly is moved in a number of passes from the headstock to the tailstock and in a plurality of passes from the tailstock to headstock. The temperature of the moving hot zone which is higher during the collapse mode than during the deposition mode softens the tube wall and allows surface tension to cause the tube to collapse into a rod. During the collapse mode, a straightening roller disclosed in U.S. Pat. No. 4,477,273 which issued on Oct. 16, 1984 in the names of B. Lynch and F. P. Partus has been used to engage the preform tube made with prior art torch assemblies to cause the resultant preform tube to be substantially straight.
Typically, torch assemblies which are used in the above-identified process for making a preform include a plurality of gas outlets which open to an arcuate surface which is disposed about a portion of the substrate tube. A flame front which is ahead of an oxygen cone and hydrogen sheath associated with each outlet and which provides the most useful heat energy for transfer to the substrate tube is substantially closer to the torch assembly than to the substrate tube.
One of the problems with the aforementioned apparatus is that portions of the metallic torch assembly oxidize as a result of the applied heat. This causes metal oxides particles to be carried from the torch assembly and deposited on and fused to the surface of the substrate tube. Such particles contaminate the resulting preform tube and may result in an undesirable number of fiber breaks during the process of drawing optical fiber from the preform.
There has been a desire to eliminate the contamination of the preform tubes with metallic particles. A solution to this problem will yield significant dividends as the elimination of such contamination should result in improved yields and longer draw lengths of optical fiber.
Seemingly, the prior art is devoid of a solution to this problem. An acceptable solution to this problem includes methods and apparatus which should be able to be used with present apparatus and which should be useable not only during the deposition process but also during the process of collapsing a preform tube to provide the preform from which optical fiber is drawn.